Pulse Doppler Radar type sensors transmit a series of time coherent pulses of energy into a directional beam for the detection of moving objects in the presence of background reflections called clutter interference. Pulse Doppler Radar type sensors are widely used for many purposes such as the military detection of space, sea, air and ground targets; civil air traffic control; meteorology; metrology; medical diagnosis and imaging; and perimeter security motion sensors. In many of these uses, the signal may be obscured by sensor noise and/or clutter interference noise. In real environments, the sensor noise is often the smaller contributor and arises from the thermal noise present in all sensor systems. The clutter interference noise arises from undesired reflections of the transmitted energy from non-target scatterers residing somewhere within the interrogation beam. These non-target scatterers can be discrete (such as the face of a building) or distributed (such as reflections from the droplets in a cloud), and in reality are usually a combination of both (ground terrain for example).
When the desired object's reflected signal is small and/or the clutter interference noise is large, Pulse Doppler Waveforms (PDW) are often used to increase the Signal To Noise Ratio (SNR) and simultaneously increase the Signal To Clutter Ratio (SCR) (i.e. reduce the clutter interference noise). The PDW consists of a series of phase coherent pulses which are generally transmitted on short equal time intervals into a stationary or pseudo-stationary narrow beam. The time period between pulses need not be equal (although this greatly simplifies the implementation) but the timing must be known precisely.
The pulses are coherently integrated in a linear time-invariant filter, usually implemented as a Bank of Doppler Filters (BDF), specifically designed to both increase the SNR and to suppress the Clutter interference noise. The SNR is increased in a BDF because the coherently integrated signal energy increases as N2 (the number of received and coherently integrated pulses), whereas the thermal noise energy only increases nominally as N leading to a Signal to Noise power ratio increase of N. The clutter interference noise is significantly reduced because the clutter return is coherently cancelled in each Doppler Filter. Some residual clutter interference noise is always still present in the band pass of the Doppler Filters because of imperfect coherence, often dominated by the system's Phase Noise and associated frequency spectrum. However, since there are usually about as many Doppler Filters in the BDF as there are pulses, the total residual clutter interference noise is divided among the Doppler Filters, thereby somewhat further reducing the clutter interference noise. More specifically, the residual clutter interference noise can be derived from the frequency spectrum of the Phase Noise and the spectrum of the clutter interference noise within the band pass of each Doppler Filter.
Linear time-invariant filters such as the Doppler Filter generally require initialization at start-up in order to avoid undesirable transients which might tend to obscure the desired signal. In order to prevent such transients, such filters are usually operated in a steady state mode. However, steady state operation implies that a transient start-up phase occurred at a significant time prior to the start of the desired coherent integration period containing the pulses to be filtered. In non-continuously pulsed radars this start-up phase is implemented by preceding the desired coherently integrated pulse sequence with a set of “Fill Pulses” of a like pulse width, pulse shape, pulse spectrum and nominally inter-pulse interval as the pulses to be coherently integrated. The presence of the Fill Pulses initializes the filter by filling the entire range extent presented to the radar with coherent pulse energy. By filling the entire range extent with pulses, this allows time for the filter to achieve a steady state before the pulses to be coherently integrated are applied to the filter. As a result, the coherently integrated pulses will be filtered without significant debilitating transients. Note that the transients of concern here are those induced by reflections of the first few pulses from scatterers in the first ambiguous range intervals. Reflections from further out range intervals are usually not of consequence, since clutter rolls off with range due to the earth limb horizon (usually about 40 km for a ground based radar on flat terrain or ocean surface).
The problem with the aforementioned Fill Pulses is that they consume Radar energy and radiation ON-time, but do not contribute to the coherent integration or thence the sensitivity of the Radar (other than indirectly through the suppression of the aforementioned transients). Furthermore, the transmission of Fill Pulses can consume a significant fraction of the PDW Dwell Time, thereby robbing beam time from the surveillance raster of the Radar as well. Since each beam Dwell position requires a finite time period, longer Dwells Times incurred by the addition of Fill Pulses can increase the time needed to search a required solid angle volume. Additionally, track revisit rates are reduced by requiring fill pulses which degrades tracking accuracy and robustness. Finally, the need for Fill Pulses increases the prime power requirement of the radar and also forces the radar to emit more energy which can help attract Anti-Radiation Missiles (a significant survivability threat to military Radars). As a result of all these factors, there is a clear desire to either eliminate Fill Pulses altogether if possible, or at least to use their energy to contribute to the sensor's performance. The present invention focuses on recovering the Fill Pulses to contribute to the sensor's performance. It can likewise be used to reduce the prime power requirement and the emitted RF energy while retaining the same detection sensitivity.